not very savant, there 'technosavant'. First of all you do not have to take off your optic with an ACR, the upper stays the same. Secondly, even if you had a 7.62 change-out for a AR-15, you would STILL need to re-zero your optic for the change in caliber. Also lets see how long your 5.56 AR-15 lower holds up under 7.62. I have never seen or heard of a lower designed for 5.56 that can stand up to repeated heavy 7.62 usage under combat conditions
The whole POINT of modular rifles like the ACR or the FN SCAR are for the military (adaptive COMBAT rifle), not random gun nuts. I have in depth knowledge of both platforms as ive used the SCARS professionally and i own an ACR. While i agree its a annoyance with the SCAR, switching out the lower, barrel, bolt assembly and guide rod, there is still a purpose. With a few quick conversions all available from the same manufacturer (thus ensuring compatibility) you can change a micro 5.56 CQB platform into a long range sniper weapon. And on top of that, all your accessories mounted and secured to your rails can stay in place. With the 6.8 for the ACR, it requires even less parts and less work. While the 6.8 isnt as powerful or as wide-used as 7.62, the lower caliber allows that ACR frame to maintain integrity. someone needs to realize that the AR-15 is a decent but outdated platform and maybe look up the meaning of the word 'savant'.

If you plan to criticize someone, it is best not to start by typing a lot of nonsense and then claim professional experience you clearly lack.